As an example of an energy regeneration system in a working machine hydraulically driven such as a hydraulic shovel, there is a system in which a fluid pressure motor such as a variable capacity hydraulic motor is set in-line in a return fluid passage provided between a control valve and a tank, an input shaft of a fluid pressure pump such as a variable capacity hydraulic pump is connected to an output shaft of the fluid pressure motor via reduction gears, a supply port of a direction control valve is communicated with a discharge port of the fluid pressure port via a check valve, and one output port of the direction control valve is connected to an accumulator for pressure accumulation and the other output port is connected to a main pump circuit that supplies working fluid from a main pump to a fluid pressure actuator (see, for example, Patent Document 1).
This system supplies return fluid to the variable capacity hydraulic motor, drives the variable capacity hydraulic pump to accumulate pressure in the accumulator, supplies pressure oil of the accumulator to the main pump during actuator actuation, and regenerates energy.
There is a power regenerating mechanism that increases pressure of the pressurized oil discharged from a head end of a boom cylinder with a pump motor and accumulates the pressurized oil in an accumulator during boom lowering of a hydraulic shovel, accumulates the pressurized oil released from a swing motor driving circuit in the accumulator during acceleration and deceleration of swing, and, when the accumulator is in a saturated state, guides the pressurized oil to the pump motor and causes the pump motor to perform motor operation, to assist engine power (see, for example, Patent Document 2).
Besides, in recent years, in a working machine such as a hydraulic shovel, a hybrid system obtained by combining a hydraulic system and an electric system has been attempted. For example, a generator motor is provided in an engine driving unit, the generator motor is adopted for swing driving, an upper swing body is driven by the generator motor and brake energy is converted into electricity to charge a capacitor and/or a battery during swing braking, and accumulated electric power is used for the swing driving. The capacitor or battery is charged by the generator motor directly connected to the engine during light engine load and power assist is performed by the generator motor using the charged electric power during heavy load.
Patent Document 1: Japanese Patent Application Laid-open No. 2006-322578
Patent Document 2: Japanese Patent Application Laid-open No. 2010-084888